The present invention relates to photomasks used for producing integrated circuits of high integration density, e.g., large-scale integrated circuits (LSI), very large-scale integrated circuits (VLSI), etc., and to photomask blanks used to produce such photomasks. More particularly, the present invention relates to a halftone phase shift photomask whereby a projected image of very small size can be obtained, and also to a halftone phase shift photomask blank for producing the halftone phase shift photomask. Further, the present invention relates to methods of producing the halftone phase shift photomask and the halftone phase shift photomask blank.
Semiconductor integrated circuits, e.g., IC, LSI, VLSI, etc., are produced by repeating thin film forming processes, e.g., oxidation, CVD or sputtering, a photolithography process and a diffusion process, e.g., ion implantation. In the photolithography process, a resist is coated on a substrate to be processed, e.g., a silicon wafer, and this resist is subjected to exposure by a reduction projection stepper or other exposure systems using a photomask to form a desired pattern thereon, followed by development and etching.
With the achievement of high-operating speed and high integration of semiconductor integrated circuits, the minimum size of photoresist patterns formed by the above-described photolithography process has increasingly been demanded to become smaller. Accordingly, the demanded device patterns cannot be realized by the conventional reduction projection stepper exposure method that employs an ordinary photomask because of the resist pattern resolution limit of this method. To overpass this limit, a phase shift photomask having a novel structure and a phase shift exposure method that uses the phase shift photomask have been proposed, as disclosed, for example, in Japanese Patent Application Laid-Open (KOKAI) No. 58-173744 (1983) and Japanese Patent Application Post-Exam Publication No. 62-59296 (1987). The phase shift exposure method is a technique whereby the resolution and the depth of focus are improved by controlling the phase of exposure light passing through a phase shift pattern formed on a photomask.
Phase shift photomasks having various arrangements have been proposed. Among them, what is called halftone phase shift photomask such as those disclosed in U.S. Pat. No. 4,890,309 and Japanese Patent Application Laid-Open (KOKAI) No. 4-136854 (1992) has attracted attention from the expectation that it will soon be put to practical use, and some proposals have been made with regard to arrangements and materials of the halftone phase shift photomask, which enable an improvement in yield and a reduction in cost as a result of a reduction in the number of manufacturing steps required. For example, see Japanese Patent Application Laid-Open (KOKAI) Nos. 5-2259 (1993) and 5-127361 (1993).
The halftone phase shift photomask will briefly be explained below with reference to the accompanying drawings. FIG. 3 shows the principle of the halftone phase shift lithography, and FIG. 4 shows a conventional lithography method. FIGS. 3(a) and 4(a) are sectional views showing photomasks. FIGS. 3(b) and 4(b) each show the amplitude of light on the photomask. FIGS. 3(c) and 4(c) each show the amplitude of light on a wafer. FIGS. 3(d) and 4(d) each show the light intensity on the wafer. Reference numerals 101 and 201 denote substrates, and 202 a 100% light-shielding film. A semitransparent film 102 shifts the phase of incident light through substantially 180.degree. and has a transmittance of 1% to 50%. Reference numerals 103 and 203 denote incident light. In the conventional method, as shown in FIG. 4(a), the 100% light-shielding film 202, which is made of chromium, for example, is formed on the substrate 201, which is made of quartz (fused silica), for example, and the light-shielding film 202 is merely formed with a light-transmitting portion in a desired pattern. Accordingly, the light intensity distribution on the wafer has a gentle slope, as shown in FIG. 4(d). As a result, the resolution is degraded. In the halftone phase shift lithography, on the other hand, the light passing through the semitransparent film 102 and the light passing through the opening in the film 102 are in substantially inverse relation to each other in terms of phase. Accordingly, the light intensity at the pattern boundary portion on the wafer is 0, as shown in FIG. 3(d). Thus, it is possible to prevent the light intensity distribution from exhibiting a gentle slope. Accordingly, the resolution can be improved.
The halftone phase shift photomask has on a transparent substrate at least a region which is semitransparent to exposure light and a region which is transparent to the exposure light so that the phase difference between the light passing through the semitransparent region and the light passing through the transparent region is substantially .pi. radians. With the halftone phase shift photomask, the resolution improves and the depth of focus enlarges at holes, dots, spaces, lines, etc. on semiconductor devices. The halftone phase shift photomask is most effective when the following relation is satisfied: EQU d=.lambda./{2(n-1)}
where d is the thickness of the semitransparent film, .lambda. is the wavelength of exposure light, and n is the refractive index of the semitransparent film for the wavelength of exposure light.
It should be noted here that a phase shift photomask of a type other than halftone phase shift photomask requires at least two photoengraving processes to produce a mask pattern because the light-shielding film and the phase shifter film have different patterns, whereas the halftone phase shift photomask essentially requires only one photoengraving process because it has only one pattern; this is a great advantage of the halftone phase shift photomask.
Incidentally, the semitransparent film 102 of the halftone phase shift photomask is demanded to perform two functions, that is, phase inversion and transmittance control. To realize these two functions, the semitransparent film 102 may be arranged either in the form of a single-layer film wherein a single layer takes charge of both functions or in the form of a multilayer film wherein the two functions are assigned to respective layers. In the former case, the photoengraving process is required only once. Therefore, it is possible to make use of the advantage of the halftone phase shift photomask. In the latter case, however, two photoengraving processes must be carried out because the materials of constituent layers are different even in a case where the identical pattern is formed, as will be clear by taking a look at an arrangement in which a phase shifter layer of spin-on-glass (SOG) is used as a layer that effects phase inversion, and a chromium light-shielding layer is used as a layer that performs transmittance control. Therefore, the multilayer film arrangement results in a rise in cost and a reduction in yield. Accordingly, it has heretofore been necessary to form the semitransparent film 102 in a single-layer structure. Even when it adopts a multilayer structure, it is necessary to find a combination of a phase shifter layer material and a light-shielding layer material which enables the semitransparent film 102 to be formed by a single photoengraving process.
However, there is known no semitransparent film material that satisfies the above-described requirements and that can be used without any substantial problems from the viewpoint of the photomask producing process. Thus, it is extremely difficult to select a semitransparent film material. A film which is composed mainly of a chromium compound, proposed in Japanese Patent Application Laid-Open (KOKAI) No. 5-127361 (1993), is the only example that may satisfy the above-described requirements. However, chromium compounds largely vary in optical characteristics according to the chemical composition thereof. Therefore, in many cases, chromium compounds cannot practically be used as a semitransparent film for a halftone phase shift photomask.
In view of the above-described circumstances, it is a first object of the present invention to provide a halftone phase shift photomask which has a simple structure and enables the photoengraving process to be shortened, thereby making it possible to attain a reduction in cost and an improvement in yield, and which also enables the greater part of a production line for a conventional chromium photomask to be used as it is, and also provide a halftone phase shift photomask blank for producing the halftone phase shift photomask.
In the meantime, a halftone phase shift photomask structure such as that shown in FIG. 17 has been proposed. In the figure, reference numeral 501 denotes a quartz substrate, and 502 a chromium thin film. The chromium thin film 502 forms a semitransparent region, and a region where no chromium thin film 502 is present defines a transparent region. The mask with such a structure has the problem that it is difficult to repair a defect, although the mask can be processed, inspected and cleaned in a similar manner to the conventional process.
As a structure wherein a defect can be repaired in the conventional repairing process, a single-layer film of a metal oxide may be considered. FIG. 18 shows the transmittance spectrum in the wavelength range of 200 nm to 800 nm of chromium oxide as one example of metal oxides. The thickness d of this film has been adjusted so as to satisfy d=.lambda./{2(n-1)} for the exposure wavelength of the i-line (365 nm) of a super-high pressure mercury lamp. As will be clear from FIG. 18, the transmittance for this exposure wavelength is sufficiently low, but the transmittance for long wavelengths in the visible region, which are used for inspection, measurement, etc., is high.
Thus, in a phase shifter comprising a single layer of a metal oxide, it is possible to control the transmittance for the exposure wavelength, but the transmittance undesirably rises at the long wavelength side. Wavelengths at the long wavelength side, particularly the e-line (546 nm) of a super-high pressure mercury lamp, are used for inspection of photomasks and size measurement thereof. However, in the inspection of a mask having a single-layer phase shifter of a metal oxide, if the transmittance for the e-line exceeds 30%, inspection and size measurement cannot be performed because of a reduction in contrast between the transparent and semitransparent regions.
Further, since such a metal oxide has no electric conductivity, charge-up occurs during electron beam exposure, causing displacement of the resist pattern.
In addition, the transmittance of halftone phase shift photomasks varies according to exposure conditions and device manufacturers. Therefore, various levels of transmittance are demanded. It is difficult to control the refractive index and the extinction coefficient so that only the transmittance varies according to film forming conditions without changing the phase difference.
In view of the above-described problems of the background art, it is a second object of the present invention to provide a halftone phase shift photomask which is designed so that:
1 the rise in the transmittance for a long wavelength in the visible region, which is used for inspection, measurement, etc., is suppressed to prevent reduction in contrast between the transparent and semitransparent regions, thereby facilitating inspection and measurement; PA1 2 electric conductivity is imparted to the phase shifter film to thereby prevent occurrence of charge-up; PA1 3 the control of transmittance can be facilitated with the exposure wavelength phase difference held at 180.degree.; PA1 4 the reflectivities of the obverse and reverse surfaces can be controlled; and PA1 5 an optimal multilayer structure can be realized for each of at least two different kinds of exposure light by controlling the thickness of each layer with the film composition of each layer maintained as it is. PA1 1 the rise in the transmittance for a long wavelength in the visible region, which is used for inspection, measurement, etc., is suppressed to prevent reduction in contrast between the transparent and semitransparent regions, thereby facilitating inspection and measurement; PA1 2 electric conductivity is imparted to the phase shifter film to thereby prevent occurrence of charge-up; PA1 3 the control of transmittance is facilitated with the exposure wavelength phase difference held at 180.degree.; PA1 4 the reflectivities of the obverse and reverse surfaces can be controlled; and PA1 5 an optimal multilayer structure can be realized for each of at least two different kinds of exposure light by controlling the thickness of each layer with the film composition of each layer maintained as it is. PA1 1 a film in which the ratio of the number of chromium atoms to the number of oxygen atoms is in the range of from 100:100 to 100:300; PA1 2 a film which satisfies the condition 1 and in which the number of carbon atoms contained is not smaller than 2% of the number of chromium atoms; PA1 3 a film which satisfies the condition 2 and in which a larger number of carbon atoms are contained in a surface region within the depth of 3 nm from the film surface than in the other region; PA1 4 a film which satisfies the condition 1 and in which nitrogen atoms are contained in such a proportion that the total number of nitrogen and oxygen atoms is not larger than 350 per 100 chromium atoms; PA1 5 a film which satisfies the condition 1 and in which argon atoms are contained in such a proportion that the total number of argon and oxygen atoms is not larger than 350 per 100 chromium atoms; and PA1 6 a film which satisfies any of the conditions 1 to 5 and which contains impurity atoms other than chromium, oxygen, carbon, nitrogen and argon atoms within the range in which the refractive index for exposure light that is obtained by ellipsometry will not be changed by 0.1 or more. PA1 Wide scan: 1,000 eV to 0 eV (B. E.) PA1 Cr 2p: 620 eV to 570 eV (B. E.) PA1 O ls: 560 eV to 520 eV (B. E.) PA1 C ls: 320 eV to 270 eV (B. E.) PA1 N ls: 430 eV to 380 eV (B. E.) PA1 Carbon: 1.00 PA1 Oxygen: 2.85 PA1 Chromium: 7.60 PA1 Nitrogen: 1.77 PA1 Argon: 3.13 PA1 1 the transmittance at the long wavelength side can be held down to a relatively low level; PA1 2 the semitransparent film can be provided with charge-up preventing properties; PA1 3 the control of transmittance can be facilitated without changing the phase difference of the exposure wavelength; PA1 4 the reflectivities of the obverse and reverse surfaces can be controlled; and PA1 5 the film thickness of each layer can be controlled with the film composition of each layer maintained as it is, thereby realizing an optimal multilayer structure for each of at least two different kinds of exposure light. PA1 (1) a semitransparent film that constitutes the semitransparent region is arranged in the form of a double-layer film which includes, in order from the transparent substrate side, a single-layer film 3 of a compound selected from among chromium oxide, chromium oxide nitride, chromium oxide carbide, and chromium oxide nitride carbide, and a single-layer film 4 of either chromium or chromium nitride, as shown in FIG. 10; or PA1 (2) the semitransparent film is arranged in the form of a double-layer film which includes, in order from the transparent substrate side, a single-layer film 5 of either chromium or chromium nitride, and a single-layer film 6 of a compound selected from among chromium oxide, chromium oxide nitride, chromium oxide carbide, and chromium oxide nitride carbide, as shown in FIG. 11; or PA1 (3) the semitransparent film is arranged in the form of a triple-layer film which includes, in order from the transparent substrate side, a single-layer film 7 of a compound selected from among chromium oxide, chromium oxide nitride, chromium oxide carbide, and chromium oxide nitride carbide, a single-layer film 8 of either chromium or chromium nitride, and a single-layer film 9 of a compound selected from among chromium oxide, chromium oxide nitride, chromium oxide carbide, and chromium oxide nitride carbide, as shown in FIG. 12.
FIG. 23 is a sectional view showing one example of a halftone phase shift photomask disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 4-136854 (1992) as a conventional example of the halftone phase shift photomask relating to the present invention. The conventional photomask includes a glass substrate 411 and a patterned semitransparent film 412 provided thereon. The semitransparent film 412 is made of a spin-on-glass (SOG) having a light-absorbing material added thereto.
However, SOG generally has a low bond strength with respect to the substrate in comparison to a film that is formed by physical vapor deposition (PVD), e.g., sputtering, which is used for ordinary photomasks. Accordingly, the film may be separated or cracked during a physical cleaning process which is commonly carried out in ordinary photomask processing by using a brush scrubber, a high-pressure jet spray, an ultrasonic cleaner, etc. Therefore, it is difficult to clean the film satisfactorily.
Further, the refractive index of SOG for the exposure wavelength of 365 nm (i-line) is generally low, i.e., of the order of 1.4 to 1.5, and it is necessary in order to effect a 180.degree. phase shift to provide SOG to a thickness of about 365 nm to 456 nm, which is larger than the thickness of light-shielding films, composed mainly of chromium or molybdenum, of ordinary photomasks, i.e., about 60 nm to 130 nm. Accordingly, SOG cannot be etched with sufficiently high accuracy, and it is difficult to obtain vertical side-walls by etching.
In view of the above-described problems of the background art, it is a third object of the present invention to provide a halftone phase shift photomask for which a physical cleaning process used for cleaning ordinary photomasks can be used as it is and which has vertical side-walls processed with high accuracy, and also provide a halftone phase shift photomask blank for producing the halftone phase shift photomask.